1. Field of the Invention
The present invention relates generally to sealed rotary rock bits and, more particularly, to the lubrication systems utilized in such bits.
2. Description of the Prior Art
A rotary rock bit generally consists of a main bit body adapted to be connected to a rotary drill string. A conventional bit usually includes two or more legs integrally connected to form a bit body. Each leg includes a cutter rotatively mounted on a journal pin extending from the leg. Bearings are provided between the cutter and the journal pin to promote rotation of the cutter and means are provided on the outer surface of the cutter for disintegrating the formations as the bit and cutter rotate.
In lubricated rock bits, a lubrication system is provided which includes an annular seal located at or near the back-face of the cutter to prevent the lubricant from leaking from the bearing area to the exterior of the rock bit and prevent drilling fluid and debris from entering the bearing area. The lubrication system further includes a reservoir filled with a lubricant, typically a high viscosity petroleum grease, with passages provided to communicate the reservoir with the bearing space between the cutter and the journal pin. A compensator in the form of a resilient membrane is located in the reservoir with one side of the membrane exposed to the lubricant and the other side exposed to the exterior of the rock bit. The compensator functions to equalize the pressure on the mud side of the seal with the pressure on its lubricant side under varying pressure conditions.
A compensator of the type described is found in U.S. Pat. No. 3,476,195. The referenced patent also describes a low pressure relief valve which functions to relieve excessive gas pressures in the bit during normal drilling. However, during the raising and lowering of the drill bit into an existing bore hole, a substantial pressure differential can develop. Providing a low pressure relief system would not operate properly because the relief valve would open prematurely before deleterious pressure differentials are reached. Therefore, too much lubricant would be lost during the drill lowering operations.
Moreover, there are severe limitations as to space in conventional rock bits and the amount of space taken up by the relief valve in the referenced patent is considered by many as being too much for the safe operation of the rock bit.